An organic electroluminescence device (hereinafter the term “electroluminescent” is often abbreviated as “EL”) is a self-emission device utilizing the principle that a fluorescent compound emits light by the recombination energy of holes injected from an anode and electrons injected from a cathode when an electric field is impressed.
Since C. W. Tang et al. of Eastman Kodak Co. reported a low-voltage driven organic EL device of stack type (Non-patent Document 1, or the like), studies on organic EL devices wherein organic materials are used as the constituent materials has actively been conducted.
The organic EL device reported by Tang et al. has a stack structure in which tris(8-hydroxyquinolinol)aluminum is used in an emitting layer and a triphenyldiamine derivative is used in a hole-transporting layer. The advantages of the stack structure are to increase injection efficiency of holes to the emitting layer, to increase generation efficiency of excitons generated by recombination by blocking electrons injected in the cathode, to confine the generated excitons in the emitting layer, and so on.
As the stack structure of the organic EL device, a two-layered type of a hole-transporting (injecting) layer and an electron-transporting emitting layer, and a three-layered type of a hole-transporting (injecting) layer, an emitting layer and an electron-transporting (injecting) layer are widely known. In such stack structure devices, their device structures and fabrication methods have been contrived to increase recombination efficiency of injected holes and electrons.
As a hole-transporting material used in an organic EL device, an aromatic diamine derivative described in Patent Document 1 and an aromatic condensed-ring diamine derivative described in Patent Document 2 have been known.
However, in order that an organic EL device using these aromatic diamine derivatives as a hole-transporting material has a sufficient luminance, it is necessary to increase an applied voltage. The increased voltage results in problems such as the shorter device lifetime and higher power consumption.
As a solution thereof, it is proposed that a hole-injecting layer of an organic EL device is doped with a electron-accepting compound such as a Lewis acid (Patent Documents 3 to 8, etc.). However, electron-accepting compounds used in Patent Documents 3 to 6 have the following disadvantages. They are unstable when handling during the production of an organic EL device, or they lack stability such as heat resistance, thereby shortening the lifetime of the resultant device.
Tetrafluorotetracyanoquinodimethane (TCNQF4), which is an electron-accepting compound exemplified in Patent Documents 5 and 7 to 9, is easily sublimated due to its small molecular weight, and being substituted with fluorine. Thus, they may scatter in an apparatus during the production of an organic EL device by vacuum deposition, resulting in contamination of the apparatus and device.
The invention was made in view of the problems mentioned above, and an object of the invention is to provide an electron-accepting material suitable for a constituent material of an EL device.    Patent Document 1: U.S. Pat. No. 4,720,432    Patent Document 2: U.S. Pat. No. 5,061,569    Patent Document 3: JP-A-2003-031365    Patent Document 4: JP-A-2001-297883    Patent Document 5: JP-A-2000-196140    Patent Document 6: JP-A-H11-251067    Patent Document 7: JP-A-H4-297076    Patent Document 8: JP-T-2004-514257    Patent Document 9: US 2005/0255334A1    Non-Patent Document 1: Applied Physics, Letters, 51, 913 (1987)